I did only pay for Android software twice, all other software was free.

First I had to spend 12€ for Teksoft's BlueInput software needed to use the bluetooth keyboard.
The Android version of my ZTE Blade is 2.2.
There seems to be no other vendor having implemented the needed bluetooth stack.
The software is really worth its price, this is the config screen:

The second software I had to pay 3€ for was Screenshot UX -- Android 2.2 does not have a Screenshot facility.
All screenshots from my ZTE Blade are done with it, like the previous one.

Another important software was Opera Mini web browser.
Yes, there is the Android web browser, which is fine.
But Opera Mini allows for cheaper surfing if not connected by WLAN or daily flat.

The cool app My Data Manager allows to track traffic (WLAN as well as Mobile).
Important when connected Mobile.
But also to keep track of usage with 2€/day 500MB flat if used.

There is a preinstalled software Docs To Go allowing to view PDF files.
But I really like (free) Aldiko Book Reader.

For XSLT I first used it for (free) Essential XML Quick Refernece PDF file.

And because of upcoming XQuery 1.0 and XPath 2 support of DataPower V6.0,
I do read Michael Kay's famous book (luckily that is available without cost for me as IBM employee).

Screenshot UX allows to mark stuff before saving.
Here a page from that book with marker (click for original 800x480 size):

Next for XML I have the proceedings from this years xmlpgrague conference.

After NSA opened the archives up to 1996 I did store some Cryptolog publications.

Really cool feature of Aldiko reader is that it allows not only PDF and .epub format.
After registering (for free) at Adobe the Aldiko reader allows to view Adobe DRM (.acsm) books:
It was "The Time Machine by H. G. Wells" which I got as .acsm file for free.

Deluxe Music Tuner is a free internet radio app.
The software provides free access to two music channels, while there are 6 pay channels.
The software told me about promotion code "1QXDJOHW" allowing free access until long ago.
But it still allows to access all 8 channels for free ... ;-)
[this totally depends on cell phone model and Android version, google or try out]

arte app allows to play content (7x24) from the last week for German/French TV channel arte.